Optical sensing techniques, usable in smoke detectors, can be classified as transmission and light scattering techniques. Transmission measurements in early fire detection require impractically long optical paths. Intensity of the scattered signal depends on many factors besides the number of particles per unit volume and intensity of incident light. Modest improvements of light scattering signal at low smoke densities can be achieved by optimizing wavelength, scattering angle, detector sensitivity, intensity of the incident light, and polarization state of the incident light.
Acoustic agglomeration of aerosols and colloids is a well-known technique to manage fine particulate matter in pharmaceutical, environmental and other industrial applications. Basic concept is based on forming standing wave in acoustic resonator. Acoustic (usually ultrasound) pressure forces both small and large particles to jiggle along with air molecules. However, larger particles have a larger slip factor and are not able to follow air movement (this is particularly true at ultrasonic frequencies) as well as smaller particles can. As a result, aerosol particles experience increased collision frequency as compared to collision due to thermal motion alone.
Each collision may result in coagulation of particles where smaller particles disappear and larger particles emerge. As larger particles get formed, they tend to move to a location of one or more nodes in the acoustic field where they start to agglomerate (a phenomena called flocculation). If the field is powerful enough they tend to levitate. In sum, if a standing wave resonant acoustic field is established in a space containing a small concentration of aerosols then in a few seconds, those particles will coagulate into larger particles at nodes of the acoustic field.
Optical smoke detectors are advantageous in that they will respond to smoldering-type fires and potentially can provide early warnings thereof. Such technologies are also usually readily acceptable world wide.
There is thus a continuing need to improve performance of optical-type smoke detectors. Preferably, sensitivity could be increased without at the same time increasing incidences of false alarms.